Method and system for data rate based vertical handoff

ABSTRACT

A method and system for using data rate as a basis to allow or deny handoff from an IS-856 (e.g., EV-DO) communication system to an IS-2000 (e.g., 1xRTT) communication system. A base station detects that a hybrid terminal has received a threshold low forward link IS-856 data rate for a threshold time period, and the base station will responsively evaluate the IS-2000 system to determine if the IS-2000 system has sufficient resources to provide at least as good a data rate as the IS-856 threshold data rate. If the base station thereby determines that the IS-2000 system is a good alternative, then the base station will allow a handoff to occur when the terminal seeks to handoff to IS-2000. On the other hand, if the base station thereby determines that the IS-2000 system is not a good alternative, then the base station will not allow the handoff to IS-2000.

FIELD OF THE INVENTION

The present invention relates to wireless communications and, moreparticularly, to vertical handoff of data communication sessions, i.e.,handoff of data sessions from one air-interface protocol to anotherair-interface protocol. The invention is particularly useful tofacilitate handoff of an active data session from an IS-856 (e.g.,EV-DO) wireless coverage area to an IS-2000 (e.g., 1xRTT) wirelesscoverage area, but the invention may extend to other vertical handoffscenarios as well.

BACKGROUND

In a typical cellular radio communications system (wirelesscommunication system), an area is divided geographically into a numberof cell sites, each defined by a radio frequency (RF) radiation patternfrom a respective base transceiver station (BTS) antenna. The basestation antennae in the cells are in turn coupled to a base stationcontroller (BSC), which is then coupled to a telecommunications switchor gateway, such as a mobile switching center (MSC) or packet dataserving node (PDSN) for instance. The switch or gateway may then becoupled with a transport network, such as the PSTN or a packet-switchednetwork (e.g., the Internet).

When a mobile station (such as a cellular telephone, pager, orappropriately equipped portable computer, for instance) is positioned ina cell, the mobile station communicates via an RF air interface with theBTS antenna of the cell. Consequently, a communication path isestablished between the mobile station and the transport network, viathe air interface, the BTS, the BSC and the switch or gateway.

With the explosive growth in demand for wireless communications, thelevel of call traffic in most cell sites has increased drastically overrecent years. To help manage the call traffic, most cells in a wirelessnetwork are usually further divided geographically into a number ofsectors, each defined respectively by radiation patterns fromdirectional antenna components of the respective BTS, or by respectiveBTS antennae. These sectors (which can be visualized ideally as piepieces) can be referred to as “physical sectors,” since they arephysical areas of a cell site. Therefore, at any given instance, amobile station in a wireless network will typically be positioned in agiven physical sector and will be able to communicate with the transportnetwork via the BTS serving that physical sector.

As a mobile station moves between coverage areas, such as sectors, of awireless communication system, or when network conditions change or forother reasons, the mobile station may “hand off” from operating in onecoverage area to operating in another coverage area. In a usual case,this handoff process is triggered by the mobile station monitoring thesignal strength of signals that it is receiving in various availablecoverage areas, and the mobile station or the BSC determining when oneor more threshold criteria are met. For instance, the mobile station maymonitor signal strength in various available coverage areas and notifythe BSC when a given coverage area has a signal strength that issufficiently higher than the coverage area in which the mobile stationis currently operating. The BSC may then direct the mobile station tohand off to that other coverage area.

In some cases, more than one air interface communication protocol mightbe implemented in a given market area. For instance, as will bedescribed more below, a given market area might provide both legacy CDMAcoverage under a standard protocol such as EIA/TIA/IS-2000 Rel. 0, A orother version thereof (hereafter “IS-2000”) and also high data ratecoverage according to a standard protocol such EIA/TIA/IS-856 Rel. 0, A,or other version thereof (hereafter “IS-856”). In such a system, amobile station might not only hand off between coverage areas under acommon air interface protocol (e.g., between IS-2000 sectors) but mayalso hand off between the different air interface protocols, such asbetween IS-2000 and IS-856. Handoff between different air interfaceprotocols (or, more generally, between different access technologies) isknown as “vertical” handoff.

To facilitate vertical handoff, a mobile station operating under a givenair interface protocol might be arranged to periodically check foravailability of service under another air interface protocol. By way ofexample, a mobile station operating under IS-856 might periodicallycheck for availability of IS-2000 service. Further, when the signalstrength of the available IS-2000 service is sufficiently higher thanthe signal strength of the existing IS-856 service, the mobile stationmay hand off from the IS-856 system to the IS-2000 system, typicallywith approval from the BSC.

SUMMARY

In a conventional dual-protocol or “hybrid” system, as described above,signal strength is used as the basis to trigger vertical. A problem withusing just signal strength to trigger such handoff, however, is thatsignal strength is not necessarily determinative of how well a givensystem will perform, or how well the system will perform compared toother systems. It is possible in some cases, for instance, that a mobilestation may be receiving a sufficiently high signal strength to justifyhandoff from a first protocol to a second protocol but that the mobilestation might actually be able to communicate at a higher data rateunder the first protocol than under the second protocol.

The present invention advances over the state of the art by using datarate (possibly among one or more other parameters) as a basis to allowor deny a vertical handoff from one air interface protocol to another.

According to an exemplary embodiment of the invention, a base station ina hybrid system will detect that a mobile station has received athreshold low forward link data rate under a first protocol for athreshold time period. In response, the base station will then determinewhether the system has sufficient resources to provide at least as goodof a data rate under a second protocol as the mobile station is gettingunder the first protocol.

If the base station thereby determines that the second protocol is agood alternative, then (assuming any other conditions are met) the basestation will allow a handoff to occur when the mobile station seeks tohand off to the second protocol. On the other hand, if the base stationdetermines that the second protocol is not a good alternative (i.e.,that the system cannot support at least the same data rate under thesecond protocol that the system is currently providing under the firstprotocol), then the base station will not allow the handoff to thesecond protocol.

Using IS-856 and IS-2000 as an example of the first and second protocolsrespectively, a base station carrying out the exemplary embodiment wouldthus determine first that a mobile station is receiving a particularforward link data rate under IS-856 for a threshold period. The basestation would then determine whether sufficient resources exist toprovide the mobile station with at least as good a data rate underIS-2000. If so, then the base station will allow the mobile station tohandoff from IS-856 to IS-2000 when the mobile station request such ahandoff. Otherwise, the base station will deny the requested handoff.

These and other aspects, advantages, and alternatives will become moreapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings. Further, it should be understood that thissummary and other descriptions throughout this document are intended toexplain the invention by way of example only and are not intended torestrict the scope of the invention as claimed. For instance, structuralelements and process steps can be rearranged, combined, distributed,eliminated, or otherwise changed, while remaining within the scope ofthe invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a wireless communication systemin which an exemplary embodiment of the invention can be implemented.

FIG. 2 is another simplified block diagram of a wireless communicationsystem in which the exemplary embodiment can be employed.

FIG. 3 is a flow chart depicting functions that can be carried out inaccordance with the exemplary embodiment.

DETAILED DESCRIPTION

The present invention will be described by way of example with referenceto hybrid IS-2000/IS-856 communications. It should be understood,however, that the invention can extend to apply in other hybrid systemsas well, such as with respect to other air interface protocols forinstance.

1. CELLULAR WIRELESS PACKET DATA COMMUNICATIONS

FIG. 1 depicts an example cellular wireless communication system adaptedto provide wireless packet data communication service for a mobilestation 12. Mobile station 12 communicates over an air interface 14 witha BTS 16, which is then coupled or integrated with a BSC 18. BSC 18 isthen coupled with a PDSN 20, which provides connectivity with apacket-switched network 22 such as the Internet and/or a wirelesscarrier's private core packet-network. Sitting as nodes on network 22are, by way of example, a remote computer 24, an authentication,authorization, and accounting (AAA) server 26, and a mobile-IP homeagent (HA) 28.

With this arrangement, after being authenticated by AAA server 26, HA 28may assign an IP address for use by mobile station 12, and mobilestation 12 may then engage in packet-data communications with entitiessuch as remote computer 24, via a communication path comprising airinterface 14, BTS 16, BSC 18, PDSN 20, and network 22.

In practice, BSC 18 may serve multiple BTSs. Each BTS may then radiateto define a given cell and a plurality of cell sectors. FIG. 2illustrates this arrangement. As shown in FIG. 2, BSC 18 is coupled withthree BTSs 30, 32, 34. Each BTS is shown at the core of a respectivecircle representing a cell, and each cell is divided into threepie-pieces representing sectors. With this arrangement, a mobile stationcan operate in any of the sectors and can communicate on packet network22 via its serving BTS, and via BSC 18 and PDSN 20.

Throughout this description, the term “base station” will refer to aradio access network element such as a BTS, a BSC, or combinationBTS/BSC, for instance. A base station may include other elements and maytake other forms as well. The term “radio network controller” (RNC) mayalso be used to refer to a BSC or more generally to a base station,usually in the context of IS-856 communications.

Preferably, the invention will be carried out by a base station, such asby BSC 18 for instance. In this regard, FIG. 1 shows that BSC 18includes a processor 36 and data storage 38. Processor 36 may compriseone or more general purpose processors (e.g., microprocessors) and/orone or more dedicated, special-purpose processors (e.g., DSP chips, orthe like). Data storage 38, in turn, may comprise one or more volatileand/or non-volatile data storage components (e.g., magnetic, optical,and/or organic storage) and may be integrated or connected withprocessor 36. As further shown, data storage 38 preferably containsprogram instructions 40 that define one or more routines executable byprocessor 36 to carry out various functions described herein. (As such,the processor, data storage, and/or instructions may be considered todefine means for carrying out the various functions.)

2. LEGACY CDMA COMMUNICATIONS

In a conventional or “legacy” Code Division Multiple Access (“CDMA”)wireless network compliant with the well known industry standardsTIA/EIA/IS-95 and TIA/EIA/IS-2000, each cell employs one or more carrierfrequencies, typically 1.25 MHz each, and each sector is distinguishedfrom adjacent sectors by a pseudo-random number offset (“PN offset”).Further, each sector can concurrently communicate on multiple differentchannels, distinguished by “Walsh codes”. When a mobile station operatesin a given sector, communications between the mobile station and the BTSof the sector are carried on a given frequency and are encoded by thesector's PN offset and a given Walsh code.

Air interface communications are divided into forward linkcommunications, which are those passing from the base station to themobile station, and reverse link communications, which are those passingfrom the mobile station to the base station. In an IS-95 or IS-2000system, both the forward link and reverse link communications in a givensector are encoded by the sector's PN offset and a given Walsh code. Onthe forward link, certain Walsh codes are reserved for use to definecontrol channels, including a pilot channel, a sync channel, and one ormore paging channels, and the remainder can be assigned dynamically foruse as traffic channels, i.e., to carry user communications. Similarly,on the reverse link, one or more Walsh codes may be reserved for use todefine access channels, and the remainder can be assigned dynamicallyfor use as traffic channels.

With this arrangement, a mobile station can be arranged to engage inpacket-data communications. To do so, the mobile station first sends apacket-data origination request over the access channel and via the BTS16 and BSC 18 to an MSC (not shown). The MSC then signals back to theBSC directing the BSC to assign an air interface traffic channel for useby the mobile station, and the BSC signals to the PDSN 20. The PDSN 20and mobile station 12 then negotiate to establish a data link layerconnection, such as a point to point protocol (PPP) session. Further,the PDSN 20 sends a foreign agent challenge message to the mobilestation, and the mobile station responds with a mobile-IP registrationrequest (MIP RRQ), which the PDSN forwards to HA 28. The HA then assignsan IP address for the mobile station to use, and the PDSN passes that IPaddress via the BSC to the mobile station.

Once the mobile station has a radio link (an assigned traffic channel),a data link, and an IP address, the mobile station is considered to bein an “active” mode. To conserve air interface resources, however, anIS-2000 system is typically arranged to release the mobile station'sassigned traffic channel after a certain amount of time during which nodata traffic passes to or from the mobile station through the PDSN. Uponrelease of the mobile station's radio link, the mobile station isconsidered to be in a “dormant” mode, in which it lacks a radio link butstill has a data link and an IP address.

While in the dormant mode, if the mobile station seeks to transmitpacket-data from its IP address, the mobile station would re-acquire aradio link by sending a new origination request message to the BSC.Similarly, if the PDSN receives packet-data destined for the IP addressof a dormant mobile station, the PDSN would notify the BSC, the BSCwould page the mobile station to cause the mobile station to thenre-acquire a radio link, and the PDSN would then pass the data along tothe mobile station.

Under IS-95 and IS-2000, a mobile station can communicate with a numberof “active” sectors at a time. (The term “active” in this context shouldnot be confused with the same term used to describe data communicationstate as noted above.) Depending on the system, the number of activesectors can be up to three or six (currently). The mobile stationreceives largely the same signal from each of the active sectors and, ona frame-by-frame basis, selects the best signal to use.

A mobile station maintains in its memory a list of the sectors in its“active” set. In addition, it maintains in its memory a list of“candidate” sectors (typically up to six), which are those sectors thatare not yet in the active set but that have sufficient signal strengththat the mobile station could demodulate signals from those sectors.Further, the mobile maintains a list of “neighbor” sectors, which arethose sectors not in the active set or candidate set but are in closevicinity to the mobile station. All other possible sectors are membersof a “remaining” set.

To facilitate a determination of which sectors should be in the mobilestation's “active” set, all base stations emit a pilot channel signal ineach sector, typically at a power level higher than other forward linksignals. A mobile station then constantly measures the strength of eachpilot that it receives and notifies a primary base station (a basestation currently serving the mobile station) when pilot strength fallsabove or below designated thresholds. The base station, in turn,provides the mobile station with an updated list of active pilots.

More particularly, in legacy systems, the base station initiallyprovides the mobile station with a Handoff Direction Message (HDM),which indicates (i) the PN offsets of the sectors in the active set and(ii) various handoff parameters that relate to pilot signal strength.Additionally, the base station initially provides the mobile stationwith a Neighbor List Update Message (NLUM), which identifies the“neighbor” sectors for the current active set.

The mobile station then monitors all of the pilot signals that itreceives, and the mobile station determines if any neighbor pilotexceeds a designated threshold strength. If so, the mobile station addsthe pilot to its “candidate” set and sends a Pilot Strength MeasurementMessage (PSMM) to the base station, indicating the estimated E_(c)/I_(o)for the pilot. Depending on current capacity and other issues, the basestation may then send an HDM to the mobile station, listing the pilot asa new member of the active set. Upon receipt of the HDM, the mobilestation then adds the pilot to its active set as instructed, and themobile station sends a Handoff Completion Message (HCM) to the basestation, acknowledging the instruction, and providing a list of thepilots (PN offsets) in its active set.

Similarly, if the mobile station detects that the signal strength of apilot in its active set drops below a designated threshold, the mobilestation starts a handoff drop timer. If the timer expires, the mobilestation then sends a PSMM to the base station, indicating theE_(c)/I_(o) and drop timer. The base station may then respond by sendingan HDM to the mobile station, without the pilot in the active set. Themobile station would then receive the HDM and responsively move thepilot to its neighbor set and send an HCM to the base station.

3. HIGH RATE PACKET-DATA COMMUNICATIONS

Under IS-2000, the highest rate of packet-data communicationstheoretically available on a fundamental traffic channel of the forwardlink is 9.6 kbps (under “Radio Configuration 3” (RC3), which provides 64Walsh codes per sector) or 19.2 kbps (under RC4, which provides 128Walsh codes per sector). In order to provide higher rate packet-dataservice to support higher bandwidth applications, the industry hasintroduced a new “high rate packet data (HRPD) system,” which is definedby industry standard IS-856.

IS-856 leverages the asymmetric characteristics of most IP traffic, inwhich the forward link typically carries a heavier load than the reverselink. Under IS-856, the forward link uses time division multiplexing(TDM), in order to allocate all power in a sector to a given user at anymoment, while the reverse link retains largely the traditional IS-2000code division multiplexing (CDM) format, albeit with the addition of a“data rate control” (DRC) channel used to indicate the supportable datarate and best serving sector for the forward link. The end result isthat a mobile station operating under IS-856 can, in theory, receivepacket-data at a rate of at least 38.4 kbps and up to 2.4 Mbps.

The IS-856 forward link is divided into time slots of length 2048 chips,and each time slot is further time division multiplexed to carry variouschannels, including a pilot channel, a Medium Access Control (MAC)channel, and, if any traffic exists, a forward traffic channel and a“control” channel. As in IS-2000, each sector in IS-856 is defined by aPN offset, and the pilot channel carries an indication of the sector'sPN offset. Also as in IS-2000, a mobile station operating under IS-856monitors the pilot signal emitted by various sectors as a basis tofacilitate active set management, i.e., to facilitate handoff from onesector to another.

IS-856 introduces a “virtual soft handoff” concept, in which a mobilestation communicates with the network on just a “best” sector of itsactive set at any given time. As in IS-2000, the mobile station monitorsthe signal strength of pilot signals emitted by various sectors, and themobile station uses threshold handoff parameters such as those notedabove as a basis to trigger the addition of a sector to the mobilestation's candidate set. Further as in IS-2000, the mobile station thensends a revised candidate set to the network, and the network decideswhether to revise the mobile station's active set. If the networkdecides to update the mobile station's active set, the network sends anHDM to the mobile station and further instructs each sector tocommunicate with the mobile station.

Unlike IS-2000 in which forward traffic information is routed to allsectors in the mobile station's active set, however, forward trafficunder IS-856 is routed to only a “best” sector that the mobile stationselects from its active set, typically the sector that has the strongestpilot signal (or, more particularly, the highest signal to noise ratio(C/I)). In practice, the mobile station will monitor the pilot signalsof the sectors in its active set, and will include in its DRC (on thereverse link) an indication of the selected serving sector. Only thatsector will then serve the mobile station with forward link packets.Thus, a mobile station may quickly hand off from one IS-856 servingsector to another by simply instructing the base station which sector itintends to communicate with.

Further, the mobile station will select a data rate based on the C/Ithat it measures for the selected sector, and the mobile station willinclude in its DRC a request to receive forward link communications atthat data rate. According to IS-856, the higher the C/I, the higher therequested data rate, and the lower the C/I, the lower the requested datarate.

Upon receipt of the DRC from the mobile station, the base station willdecide whether or not it will grant the mobile station's requestedforward link data rate. In this regard, the base station will apply ascheduling algorithm to determine which requests to grant and whichrequests to deny. The scheduling algorithm is vendor-specific and cantherefore take various forms. By way of example, one schedulingalgorithm may provide for granting the best data rate requested bymobile stations currently requesting data rate. Another schedulingalgorithm (the “proportional fairness algorithm”) provides for the basestation maintaining an average data rate provided to a mobile stationover time, and granting a request to the mobile station for which thedifference between its requested data rate and its average data rate isthe greatest. Other examples are possible as well. If the base stationdecides to grant the request, the base station will then deliver data tothe mobile station at the requested rate (although, in some cases, theactually delivered forward link data rate may differ from the raterequested and granted).

To acquire packet data connectivity under IS-856, after a mobile stationfirst detects an IS-856 carrier, the mobile station sends to its BSC (or“RNC”) 18 a UATI (Universal Access Terminal Identifier) request, andreceives in response an International Mobile Station Identifier (IMSI),which the mobile station can then use to identify itself in subsequentcommunications with the BSC. The mobile station then sends aconnection-request to the BSC 18, and the BSC responsively invokes aprocess to authenticate the mobile station and to have the mobilestation acquire a data link.

In particular, the BSC 18 sends an access request to an Access NetworkAAA (ANAAA) server (which may be different than the AAA server 26 shownin FIG. 1), and the ANAAA server authenticates the mobile station. TheBSC 18 then assigns radio resources for the data session, by directingthe mobile station to operate on a particular time slot traffic channelon the forward link and a particular Walsh coded traffic channel on thereverse link. Further, the BSC signals to the PDSN 20, and the PDSN andmobile station then negotiate to establish a PPP data link. In addition,as in the IS-2000 process, the mobile station then sends an MIP RRQ tothe PDSN, which the PDSN forwards to the HA 28, and the HA assigns amobile-IP address for the mobile station to use.

As in IS-2000, once the mobile station has acquired an IS-856 radiolink, a data link, and an IP address, the mobile station is consideredto be in an active mode. In the active mode, the mobile station is givenfull use of the sector power in its assigned time slot, i.e., each timeits time slot occurs, which facilitates higher rate data communication.Further, as in IS-2000, if the mobile station does not send or receivedata for a defined period of time, the mobile station enters a dormantmode. In the dormant mode, an IS-856 system maintains the logical stateof the mobile station's session (e.g., IP address, PPP state, and radiolink session information), while releasing the mobile station's radiolink resources (e.g., the time slot that had been assigned for use bythe mobile station). With the maintained session state, the mobilestation can quickly re-acquire a radio link so as to send or receivepacket data, thereby giving the appearance that the mobile station is“always on.”

4. HYBRID IS-2000/IS-856 SYSTEMS

Given the proliferation of legacy IS-2000 systems, IS-856 was designedto be backwards compatible and to facilitate “hybrid” operation. A“hybrid access terminal,” in this scenario, is defined as a mobilestation that can operate on both IS-2000 and IS-856 networks. A typicalhybrid terminal, for instance, may be capable of receiving voice, shortmessage service (SMS) messages, and dedicated channel data services onIS-2000 networks, as well as high-speed packet data service on IS-856networks.

In a hybrid system, IS-2000 and IS-856 coverage are provided ondifferent carrier frequencies, each of which is 1.25 MHz in bandwidthfor consistency with legacy operation. Further, most IS-856 systems areprovided as overlays on existing IS-2000 systems, such that a given BSCprovides either IS-2000 coverage or both IS-2000 and IS-856 coverage.For an IS-2000 sector, the BSC may have just an IS-2000 circuit card.For a hybrid IS-2000/IS-856 sector, on the other hand, the BSC may haveboth an IS-2000 circuit card to facilitate IS-2000 operation and anIS-856 circuit card to facilitate IS-856 operation. A typical hybriddevice will be programmed to use an IS-856 data connection when facedwith the choice between IS-856 and IS-2000, i.e., when in a hybridsector.

By standard design, when a hybrid terminal is operating in an IS-856system, it will also periodically monitor the control channels in theIS-2000 system, in search of any incoming voice calls, SMS messages, orthe like, and to monitor IS-2000 pilot signals. In the active IS-856mode, the terminal periodically (e.g., every 5 seconds) tunes to thefrequency of the IS-2000 system to monitor the IS-2000 control channelsand then tunes back to the IS-856 frequency to resume the active datasession. In the dormant/idle IS-856 mode, the terminal operates in aslotted manner to monitor both the IS-856 control channel and theIS-2000 control channels.

Further, because IS-856 is typically provided as an overlay on anexisting IS-2000 system, it is possible that a hybrid terminal may needto hand off from a hybrid sector (providing both IS-2000 and IS-856coverage) to an IS-2000 sector providing no IS-856 coverage), e.g., asit moves physically out of a hybrid sector and into an IS-2000 sector.To be able to facilitate such a handoff, when a hybrid terminal isoperating in IS-856 it will simultaneously maintain an IS-2000 activeset (as will the IS-2000 BSC) as described above. In particular, as theterminal periodically tunes to the IS-2000 frequency, it will monitorthe IS-2000 pilots, consider the various threshold parameters describedabove, send PSMM messages as applicable to the IS-2000 BSC, and receiveHDM messages as applicable from the IS-2000 BSC. Thus, when the terminalseeks to hand off from IS-856 to IS-2000, the terminal can readilyswitch over to operating on one or more of the sectors currently in itsIS-2000 active set.

In a hybrid system, handoff from IS-856 to IS-2000 is generallytriggered by an analysis of power the C/I ratios of the available IS-856sectors. In particular, a hybrid terminal will, as noted above,regularly monitor the C/I of available sectors. If the terminal detectsthat the lowest available C/I is lower than a threshold “PILOT-DROP”level, then the terminal will leave the IS-856 system and beginoperating in the IS-2000 system.

If this IS-856 to IS-2000 handoff occurs when the terminal is in anidle/dormant IS-856 mode, then the terminal will simply register in theIS-2000 system and drop its IS-856 radio link. That is, the terminal mayrequest packet data connectivity in the IS-2000 system and consequentlyobtain an IS-2000 radio link. Further, because the same PDSN andmobile-IP home agent likely serves both the IS-2000 and IS-856 systems,the terminal may simply maintain its existing PPP session and mobile-IPaddress. As one way to accomplish this in practice, when the terminalacquires an IS-2000 radio link and sends a MIP RRQ to the PDSN, theterminal can include its already-assigned mobile-IP address within theMIP RRQ. When the PDSN receives the MIP RRQ and detects that theterminal already has a mobile-IP address, the PDSN would forego sendingthe MIP RRQ to HA 28. Instead, the PDSN would responsively work with theIS-2000 BSC (e.g., a packet control function (PCF) of the BSC) to set upa new radio-packet (R-P) tunnel for the IS-2000 connection and wouldtear down the R-P tunnel that it had with the IS-856 BSC (PCF).

If the IS-856 to IS-2000 handoff occurs when the terminal is in anactive IS-856 mode, on the other hand, the terminal will first switch toan IS-856 dormant mode and will then acquire dedicated packet-dataconnectivity in the IS-2000 system in the manner described in thepreceding paragraph.

It is also possible that a hybrid terminal operating on an IS-2000system may hand off from an IS-2000 sector to a hybrid sector. In thisregard, when a hybrid terminal is in an idle/dormant IS-2000 state, theterminal will perform periodic off-frequency searches in an effort todiscover existing IS-856 systems. If the terminal thereby finds anIS-856 system, it will acquire IS-856 packet-data connectivity and thenoperate in the IS-856 system as described above. Existing standardsgenerally do not permit a hybrid terminal that is in an active IS-2000mode to hand off to IS-856, mainly because, in the active IS-2000 mode,no provision is made for discovering co-existing IS-856 systems.However, once the terminal switches to an IS-2000 dormant mode, it maythen lock onto an IS-856 system as described above.

When in an idle IS-2000 mode, if a hybrid terminal locks onto an IS-856system and acquires packet-data connectivity, the terminal will applythe conventional packet-data connection process, including acquiring anIS-856 radio link, and a PPP link and IP address. When in a dormantIS-2000 mode, the terminal will also apply the conventional packet-dataconnection process, but, as with a handoff from IS-856 to IS-2000, theterminal may only need to acquire an IS-856 radio link, as the PDSN mayrecognize that the terminal already has an assigned IP address.

5. DATA-RATE BASED VERTICAL HANDOFF

As noted above, the present invention advances over the state of the artby using data rate (possibly among one or more other parameters) as abasis to allow or deny a vertical handoff. FIG. 3 is a flow chartdepicting operation of an exemplary embodiment of the invention inrelation to handoff from IS-856 to IS-2000 by way of example.

At step 50, the example process begins with a hybrid terminal operatingunder IS-856, e.g., in an active mode. At step 52, a base station (orother entity) detects that the hybrid terminal has received a thresholdlow forward link IS-856 data rate for a threshold time period. Inresponse, at step 54, the base station evaluates the IS-2000 system todetermine if the IS-2000 system has sufficient resources to provide atleast as good data rate as the terminal is getting (or has received)under IS-856. If the base station thereby determines that the IS-2000system is a good alternative, then, at step 56, the base station willallow a handoff to occur when the terminal seeks to handoff to IS-2000.On the other hand, if the base station thereby determines that theIS-2000 system is not a good alternative (i.e., it cannot support atleast the same data rate that the IS-856 system is currently providing),then, at step 58, the base station will not allow the handoff to IS-2000when the terminal seeks such handoff.

As shown by the flow chart, the process of determining whether to allowhandoff from IS-856 to IS-2000 can be carried out in advance of a thehybrid terminal requesting such handoff. While that is the preferredembodiment, it is also possible that the process of determining whetherto allow the handoff can be carried out in response to the hybridterminal requesting the handoff. For instance, once the base stationreceives a handoff request, the base station may then analyze itsrecords to determine what data rate was provided to the terminal underIS-856 over a past designated time period (e.g., the dwell period) andmay then determine whether IS-2000 can support at least that same datarate.

The following sections will discuss each of these exemplary processsteps of FIG. 3 in more detail. As noted above, however, it should beunderstood that variations on these steps and on the process as a wholeare possible within the scope of the claimed invention.

a. Detect Threshold Low Data Rate Condition Under Current Protocol

In a preferred embodiment, the base station will first detect that ahybrid terminal's forward link data rate under IS-856 falls below adesignated data rate threshold. To carry out this function, the basestation will regularly monitor the amount of data the base station istransmitting per unit time to the terminal, e.g., as a rolling average.(As noted above, under IS-856, this forward link data rate could varysomewhat from the data rate that the terminal requested.) Further, thebase station will regularly compare this rolling average data rate tothe predefined threshold, to determine when the data rate is lower thanthe threshold.

The data rate threshold can be set in various ways. In a preferredembodiment, for instance, the data rate can be set by optimization on asector by sector basis, such as by analyzing actual use over time in asector and using the average actual use as a basis to set the thresholddata rate for the sector. As a specific example, the data rate thresholdmight be set at 76.8 kbps, or 38 kbps. Other examples are possible aswell.

In response to detecting that a terminal's forward link data rate islower than the data rate threshold, the base station will thenpreferably begin decrementing a dwell timer set at some predefined timeperiod. (The specific time period is a matter of design choice.) If theterminal's forward link data rate remains below the data rate thresholdfor the entire dwell period (possibly allowing for some hysterisis toaccount for brief movements above the threshold), then the base stationproceeds to the next step. Alternatively, if the terminal's forward linkdata rate rises above the threshold, the process may end.

The main purpose of waiting for the terminal's forward link data rate toremain below the data rate threshold for the dwell period is to ensurethat the terminal is in fact experiencing a low data rate condition.Other ways to draw this conclusion include determining that theterminal's forward link data rate was below the threshold for athreshold percentage of the dwell period, or a certain number of timesduring the dwell period. Further, while the preferred embodiment focuseson forward link data rate, it should be understood that reverse linkdata rate could be considered instead of, or in addition to, forwardlink data rate.

b. Evaluate Whether Sufficient Data Rate can be Provided Under TargetProtocol

Once the base station has determined that the terminal's forward linkdata rate remained below the data rate threshold for a sufficient periodof time, the base station will next preferably analyze the IS-2000system to determine whether the IS-2000 system has sufficient resourcesavailable to support at least a desired forward link data rate. In thepreferred embodiment, the desired forward link data rate is the IS-856threshold data rate. In an alternative embodiment, however, the desiredforward link data rate could be a rate that is higher than the thresholddata rate. Still alternatively, the desired forward link data rate couldbe any designated (e.g., operator defined) data rate, so that the basestation would determine whether the IS-2000 system can support at leastthat designated data rate.

In a preferred embodiment, the function of checking for sufficientIS-2000 resources to support at least a desired data rate will involve(i) determining if the IS-2000 system has a sufficient number of Walshcodes available to provide at least the desired data rate, (ii)determining if the IS-2000 system has sufficient power available toprovide at least the desired data rate, (iii) determining if the IS-2000system has sufficient channel elements available to provide at least thedesired data rate, and/or (iv) determining if the IS-2000 system hassufficient number of packet-pipes to support at least the desired datarate.

Preferably, the base station will conclude that the IS-2000 system cansupport a sufficient data rate to justify a handoff only if at least thefirst three of these determinations are made, and the base station willotherwise conclude that the IS-2000 system cannot support a sufficientdata rate to justify the handoff. However, the base station mayalternatively condition its conclusion one any one or more of these orother determinations.

In the preferred embodiment, the first determination the base stationwill make is, for each sector in the terminal's IS-2000 active set,whether the sector has a sufficient number of Walsh codes available tosupport at least the IS-856 threshold data rate. In this regard, anIS-2000 sector can support various discrete data rates by carrying dataon a fundamental traffic channel and perhaps one or more supplementaltraffic channels. The fundamental channel uses one Walsh code, and eachsupplemental channel uses one or more Walsh codes. In particular, underRC3, (i) a data rate of 38.4 kbps requires the fundamental channel Walshcode plus 4 supplemental channel Walsh codes, (ii) a data rate of 76.8kbps requires the fundamental channel Walsh code plus 8 supplementalchannel Walsh codes, and (iii) a data rate of 153.6 kbps requires thefundamental channel Walsh code plus 16 supplemental channel Walsh codes.(Under RC4, half this many Walsh codes are needed.) Given the thresholdIS-856 data rate, the base station can thus determine for each sector inthe terminal's active set if a sufficient number of available (currentlyunused) Walsh codes exist to support at least that data rate.

For example, if the IS-856 threshold data rate is 100 kbps, then thebase station may evaluate each sector in the terminal's active set todetermine if the sector has at least 17 available Walsh codes, in orderto provide a rate that is at least 100 kbps—namely, the next quantumdata rate available with Walsh coded channels, 153.6 kbps (with onefundamental channel Walsh code and 16 supplemental channel Walsh codes).If so, the base station will conclude that the IS-2000 sector is acandidate to serve the terminal. On the other hand, if not, the basestation will conclude that the IS-2000 is not a candidate to serve theterminal.

Following this analysis for all of the sectors (if more than one) in theterminal's IS-2000 active set, if the base station concludes that atleast one of the IS-2000 sectors is a candidate to serve the terminal,the base station will proceed to the next step. Otherwise, the basestation will set a flag in data storage 38 indicating that the sector isnot a candidate to serve the terminal, i.e., that the terminal would bebetter off to remain in the IS-856 system, even though the forward linkdata rate in the IS-856 system is undesirably low. (Such a flag can beconsidered to indicate directly or indirectly that an insufficient datarate is available under the target IS-2000 system, i.e., that theIS-2000 system cannot support the desired data rate.)

Next, for each IS-2000 sector that the base station concludes is acandidate to serve the terminal, the base station will preferablyevaluate the available power in the sector, to determine if the sectorhas enough power to support the desired data rate (i.e., at least ashigh as the IS-856 threshold data rate). In this regard, it is wellknown that an IS-2000 sector allocates power dynamically to terminalsthat it serves. Vendor-specific power allocation algorithms aretypically employed for this purpose. In the preferred embodiment, thebase station may use the applicable power-allocation algorithm todetermine if sufficient power amplifier (PA) power is available tosupport the desired data rate.

By way of example, the base station could be programmed with correlationdata (set as a matter of engineering design, or set dynamically) thatcorrelates particular data rates with particular levels of power. Giventhe desire to support a particular data rate, the base station can thenrefer to the correlation data to determine what power would be necessaryor desired to support the data rate, and the base station can thendetermine whether the sector can allocate at least that amount of power,given the power being used currently to serve users in the sector.Through this or another analysis, if the base station concludes that thesector does not have sufficient power to support the desired data rate,then the base station will set a flag in data storage 38 indicating thatthe sector is not a candidate to serve the terminal.

Further, for each IS-2000 sector that the base station concludes is acandidate to serve the terminal (e.g., for each one determined to havesufficient number of available Walsh codes and a sufficient amount ofpower available to provide at least the IS-856 threshold data rate), thebase station will determine whether the sector has a sufficient numberof channel elements available to support at least the desired data rate.As is well known, channel elements are the logical components at a basestation to support communications. Mechanisms can be provided todynamically allocate channel elements, so this may be a non-issue insome instances. However, in other instances, the number of channelelements may be an equally significant consideration.

With this preferred analysis, if the base station determines that agiven IS-2000 sector has sufficient power and sufficient number ofchannel elements, then, given that the sector also has a sufficientnumber of Walsh codes, the base station will set a flag in data storage38 indicating that the sector is a candidate to serve the terminal. Onthe other hand, if the base station thereby determines that the sectordoes not have sufficient power and/or that the sector does not have asufficient number of channel elements, then the base station will set aflag in data storage 38 indicating that the sector is not a candidate toserve the terminal, i.e., that the terminal would be better off toremain in the IS-856 system, even though the forward link data rate inthe IS-856 system may be undesirably low.

c. Granting or Denying Vertical Handoff

According to the preferred embodiment, this entire process will beoccurring at the base station at the same time as the terminal isconventionally evaluating the IS-856 pilot to determine whether theIS-856 pilot falls below the PILOT-DROP level. As noted above, when theterminal detects that the IS-856 pilot falls below the PILOT-DROP level,the terminal will initiate a handoff to IS-2000. In accordance with theexemplary embodiment, when the terminal seeks to perform the handoff,the base station will then use the valuable information it gleans fromthe inventive process to determine whether or not to allow the handoffto proceed.

In this regard, it is understood that a terminal seeking to acquirepacket data connectivity under IS-2000 may first acquire a fundamentalchannel and may then acquire one or more supplemental channels so as tosupport a desired data rate; at issue in the present process may bewhether the base station will allow the terminal to gain packet dataconnectivity under IS-2000 at all. If insufficient resources exist underIS-2000 to support the desired data rate, then the base station maypreclude the handoff, even with respect to the initial acquisition of afundamental channel. Whereas, if sufficient resources exist, then thebase station may allow the handoff to occur.

Thus, in the preferred embodiment, if the base station has concludedthat no IS-2000 sector in the terminal's active set can support asufficient data rate (i.e., at least as high a data rate as the terminalis receiving under IS-856), then the base station may decline to grantthe terminal's IS-2000 packet data origination request. On the otherhand, if the base station has concluded that at least one IS-2000 sectorin the terminal's active set can support a sufficient data rate, thenthe base station may grant the terminal's IS-2000 origination request.In that case, the base station may direct the terminal to operate on theIS-2000 sector that can support the highest data rate. (In systems thatsupport communication on more than one sector at a time, the basestation could direct the terminal to operate on those sectors thatsupport the highest data rate.) The process of a base station directinga terminal to operate on a particular sector is well known and thereforenot described here.

As noted above, the base station could carry out the above process ofevaluating IS-2000 system resources before, during and/or after theterminal seeks to initiate a handoff to IS-2000. Preferably, however,the base station will evaluate the availability of Walsh codes, power,and channel elements within a period of 300 milliseconds to 1 second ofwhen the terminal seeks to initiate a handoff, so that the informationgleaned will be fresh and likely accurate. If more than 1 second haspassed since the base station last evaluated the IS-2000 resources forthe terminal, the base station may simply allow the handoff or may takesome other action.

If, after this process, the terminal proceeds with handoff to theIS-2000 system, the base station can assign the traffic channels, power,and channel elements necessary to support the data that is to betransmitted to the terminal (e.g., data buffered at the base station).Similarly, the terminal can request the base station to assign trafficchannels sufficient to support the desired data rate on the reverselink.

6. CONCLUSION

An exemplary embodiment of the present invention has been describedabove. Those skilled in the art will understand, however, that changesand modifications may be made to this embodiment without departing fromthe true scope and spirit of the present invention, which is defined bythe claims.

1. A method comprising: detecting that a hybrid wireless terminal hasreceived a data rate below a threshold data rate while operating under afirst air interface protocol; and conditioning handoff of the terminalfrom the first air interface protocol to a second air interface protocolon a determination that a desired data rate is available under thesecond air interface protocol, wherein conditioning handoff of theterminal from the first air interface protocol to a second air interfaceprotocol on a determination that a desired data rate is available underthe second air interface protocol comprises: making the determinationthat the desired data rate is available under the second air interfaceprotocol, wherein making the determination that the desired data rate isavailable under the second air interface protocol comprises (i)determining that, under the second air interface protocol, a sufficientnumber of Walsh codes exist to support the desired data rate, (ii)determining that, under the second air interface protocol, sufficientradio link power exists to support the desired data rate, and (iii)determining that, under the second air interface protocol, a sufficientnumber of channel elements exists to support the desired data rate. 2.The method of claim 1, wherein the first air interface protocol isIS-856 and the second air interface protocol is IS-2000.
 3. The methodof claim 1, wherein the threshold data rate is a forward link data rate,and wherein the desired data rate is a forward link data rate.
 4. Themethod of claim 1, wherein detecting that the terminal has received adata rate below the threshold data rate while operating under the firstair interface protocol comprises detecting that the terminal hasreceived a data rate below the threshold data rate for at least athreshold time period while operating under the first air interfaceprotocol.
 5. The method of claim 1, wherein detecting that a hybridwireless terminal has received a data rate below the threshold data ratewhile operating under a first air interface protocol comprises:determining a given data rate at which the terminal has operated underthe first air interface protocol; and comparing the given data rate withthe threshold data rate, wherein the desired data rate is the thresholddata rate.
 6. In a hybrid cellular wireless communication system of thetype providing both IS-2000 coverage and IS-856 coverage, wherein a basestation serves a hybrid terminal that is capable of operating underIS-2000 and IS-856, a method performed by the base station comprising:detecting that the hybrid terminal has received an IS-856 forward linkdata rate less than a threshold data rate for a threshold period oftime; responsively evaluating IS-2000 resources to make a determinationof whether an IS-2000 forward link data rate of at least as high as thethreshold data rate is available for the hybrid terminal; if thedetermination is that an IS-2000 forward link data rate of at least ashigh as the threshold data rate is available for the hybrid terminal,then allowing the hybrid terminal to hand off from IS-856 to IS-2000;and if the determination is that an IS-2000 forward link data rate of atleast as high as the threshold data rate is not available for the hybridterminal, then preventing the hybrid terminal from handing off fromIS-856 to IS-2000.
 7. The method of claim 6, wherein the hybrid terminalhas an IS-2000 active set of one or more sectors, and wherein evaluatingIS-2000 resources to make the determination of whether an IS-2000forward link data rate of at least as high as the threshold data rate isavailable for the hybrid terminal comprises: determining respectivelyfor each sector in the hybrid terminal's IS-2000 active set whether thesector has a sufficient number of Walsh codes available to support adata rate of at least as high as the threshold data rate.
 8. The methodof claim 6, wherein the hybrid terminal has an IS-2000 active set of oneor more sectors, and wherein evaluating IS-2000 resources to make thedetermination of whether an IS-2000 forward link data rate of at leastas high as the threshold data rate is available for the hybrid terminalcomprises: determining for each sector in the hybrid terminal's IS-2000active set whether the sector has sufficient power to support a datarate of at least as high as the threshold data rate.
 9. The method ofclaim 6, wherein the hybrid terminal has an IS-2000 active set of one ormore sectors, and wherein evaluating IS-2000 resources to make thedetermination of whether an IS-2000 forward link data rate of at leastas high as the threshold data rate is available for the hybrid terminalcomprises: determining for each sector in the hybrid terminal's IS-2000active set whether the sector has sufficient channel elements to supporta data rate of at least as high as the threshold data rate.
 10. Themethod of claim 6, wherein the hybrid terminal has an IS-2000 active setof one or more sectors, and wherein evaluating IS-2000 resources to makethe determination of whether an IS-2000 forward link data rate of atleast as high as the threshold data rate is available for the hybridterminal comprises: concluding that an IS-2000 forward link data rate ofat least as high as the threshold data rate is available for the hybridterminal if at least one sector in the hybrid terminal's IS-2000 activeset (i) has a sufficient number of Walsh codes available to support adata rate of at least as high as the threshold data rate, (ii) hassufficient power to support a data rate of at least as high as thethreshold data rate, and (iii) has a sufficient number of channelelements to support a data rate of at least as high as the thresholddata rate.
 11. The method of claim 10, wherein concluding that anIS-2000 forward link data rate of at least as high as the IS-856 forwardlink data rate is available for the hybrid terminal if at least onesector in the hybrid terminal's IS-2000 active set (i) has a sufficientnumber of Walsh codes available to support a data rate of at least ashigh as the threshold data rate, (ii) has sufficient power to support adata rate of at least as high as the threshold data rate, and (iii) hasa sufficient number of channel elements to support a data rate of atleast as high as the threshold data rate comprises: determiningrespectively for each sector in the hybrid terminal's IS-2000 active setwhether the sector has a sufficient number of Walsh codes available tosupport a data rate of at least as high as the threshold data rate; andfor each sector, if any, determined to have a sufficient number of Walshcodes available to support a data rate of at least as high as thethreshold data rate, determining whether the sector has sufficient powerand channel elements to support a data rate of at least as high as thethreshold data rate.
 12. The method of claim 6, further comprising: ifthe determination is that an IS-2000 forward link data rate of at leastas high as the threshold data rate is available for the hybrid terminal,then setting a flag in data storage to indicate that an IS-2000 forwardlink data rate of at least as high as the threshold data rate isavailable for the hybrid terminal; if the determination is that anIS-2000 forward link data rate of at least as high as the threshold datarate is not available for the hybrid terminal, then setting the flag indata storage to indicate that an IS-2000 forward link data rate of atleast as high as the threshold data rate is not available for the hybridterminal.
 13. The method of claim 12, further comprising: receiving arequest from the hybrid terminal to handoff from IS-856 to IS-2000;based on the flag, deciding whether or not to allow the requestedhandoff.
 14. The method of claim 13, wherein: receiving the request fromthe hybrid terminal to handoff from IS-856 to IS-2000 comprisesreceiving from the hybrid terminal an origination request seeking tooriginate a packet-data connection under IS-2000; and deciding whetherto allow or prevent the requested handoff comprises deciding whether togrant or deny the origination request.
 15. In a hybrid cellular wirelesscommunication system of the type providing wireless communicationservice under at least a first air interface protocol and a second airinterface protocol, wherein a base station serves a hybrid terminal thatis capable of operating under the first air interface protocol and underthe second air interface protocol, the improvement comprising: means fordetecting that the hybrid terminal has received a given forward linkdata rate less than a threshold data rate while the hybrid terminal wasoperating under the first air interface protocol; means for responsivelyevaluating resources under the second air interface protocol to make adetermination of whether at least a desired forward link data rate isavailable for the hybrid terminal under the second air interfaceprotocol; and means for using the determination to decide whether or notto allow the hybrid terminal to hand off from the first air interfaceprotocol to the second air interface protocol, wherein the first airinterface protocol is IS-856 and the second air interface protocol isIS-2000, and wherein the means for responsively evaluating resourcesunder the second air interface protocol to make a determination ofwhether at least a desired forward link data rate is available for thehybrid terminal under the second air interface protocol comprises meansfor determining whether, under the second air interface, (i) sufficientWalsh codes exist to support the desired forward link data rate, (ii)sufficient power exists to support the desired forward link data rate,and (iii) sufficient channel elements exist to support the desiredforward link data rate.
 16. The improvement of claim 15, wherein theimprovement is disposed at least in part within the base station. 17.The improvement of claim 15, wherein the desired forward link data rateis the threshold data rate.